The invention relates to a high pressure fuel injection system for diesel engines including a hydraulically driven piston pump and an injection nozzle. The piston pump and the injection nozzle may be combined into a single assembly in which the pump piston is driven by a servo piston of large diameter and wherein a switching valve alternately admits fluid pressure to the servo piston and connects the servo piston with a return line at low pressure. The injection nozzle includes a valve needle which is loaded in the closure direction by a spring and in addition may be loaded by servo pressure admitted through a bypass channel.
In a known fuel injection system of this type, embodied as a pump/nozzle assembly (see, for example, U.S. Pat. No. 2,916,028), the servo pressure acting on the valve needle is a type of hydraulic spring and thus affects both the opening as well as the closing pressure of the injection nozzle. In another known injection system of the type described above, (see, for example, U.S. Pat. No. 3,908,621) the closing spring of the injection nozzle is additionally affected by servo pressure whose magnitude is changeable in order to change the injection pressure of the system and thus also changes the strength of the hydraulic spring acting on the valve needle in a manner which is proportional to the servo pressure. Both of these injection systems share the disadvantage that the servo pressure acting as a supplementary hydraulic spring affects both the opening as well as the closing pressure acting on the injection nozzle, by the same amount.
Modern diesel engines subjected to heavy loads require extremely short injection times. In addition, the injection process must be capable of abrupt termination, preferably within one degree of crankshaft angle, because a delayed termination of injection and the resulting postinjection dribbling result in poor combustion and an increase in the emission of hydrocarbon and CO components. The most favorable combustion process is achieved if the injection begins at a relatively low opening pressure resulting in a short injection jet and if the pressure is increased toward its maximum value near the end of the injection with a correspondingly longest injection jet and is then abruptly interrupted. As a practical matter, a very abrupt needle valve closure is extremely difficult to realize due to the hydraulic and mechanical conditions in a fuel injection system.